Metal matrix composite having improved microstructure and the process for making the same

ABSTRACT

The microstructure of a metal matrix composite comprising copper and reinforcement material is improved when the metal matrix composite comprises at least about 0.02% by weight of at least one of silver and gold. In one embodiment, the process for making a metal matrix composite comprises compacting powder particles of a reinforcement material to form a green compact, sintering the green compact to produce a porous skeletal body and infiltrating the porous skeletal body with an infiltrant comprising copper and at least about 0.1% of at least one of silver and gold, based on the weight of infiltrant. In another embodiment, the process comprises forming a mixture of particles of reinforcement material, copper and at least about 0.02% by weight of at least one of silver and gold, compacting the mixture to form a green compact, and sintering the green compact to form a near net shape metal matrix composite. The metal matrix composite has a more uniform microstructure along its cross-section and increased density, thereby providing for improved physical and thermal properties.

FIELD OF THE INVENTION

[0001] The present invention relates to a metal matrix composite containing copper and the metal process for making the same.

BACKGROUND

[0002] A metal matrix composite is a polyphase mixture having a physically continuous metallic constituent, the metal matrix, and a reinforcement material embedded in the metal matrix. A metal matrix composite having at least two distinct phases can be designed to achieve certain overall physical characteristics such as mechanical, thermal, electrical and chemical properties by varying the relative proportions of the metal matrix and reinforcement material to satisfy end use specification requirements. As an example, a metal matrix composite that contains copper as the metal matrix and tungsten as the reinforcement metal is commonly used as a substrate in semiconductor devices in radio frequency (RF), microwave, millimeter-wave, and optoelectronic applications, and in a myriad of electronic applications that require high thermal conductivity and good dimensional stability. Using copper as the metal matrix provides for high thermal conductivity, while the tungsten reinforcement material, which has a coefficient of thermal expansion that is similar to that of the heat-generating component attached to it, provides good dimensional stability during thermal cycling.

[0003] High-volume, reproducible metal matrix composites can be made via powder metallurgy techniques using powders of the metal matrix and reinforcement material. In one process, a metal matrix composite is made by mixing powder particles of the metal matrix and reinforcement material, compacting the powder mixture to produce a green compact, and sintering the green compact to produce a near net shape metal matrix composite. A problem with this process is that a non-uniform microstructure can result if the metal matrix and the reinforcement material have poor solubility. The microstructure may display regions of both high density and high porosity.

[0004] In another powder metallurgy technique, a metal matrix composite is produced by compacting powder particles of a reinforcement material to form a green compact having a predetermined shape, sintering the green compact to form a porous skeletal body, and infiltrating the porous skeletal body with a metal matrix, such as copper, for example, to produce a metal matrix composite. The porous skeletal body is formed having a density that is substantially less than theoretical density, generally less than 90%, and in many cases less than 75%, of theoretical density, and the matrix is infiltrated into the voids and interstices of the porous skeletal body, thereby forming a completed metal matrix composite having a density of at least 97% of theoretical density. A problem with this process, again due to poor solubility of the materials, is that the metal matrix composite produced at the completion of the infiltration step can display areas devoid of the metal matrix or areas of pooled metal matrix, or both. The pooled metal matrix can be seen on the surface of the finished composite, and the areas devoid of metal matrix can be apparent when the metal matrix composite is machined and the interior portion of the composite is exposed.

[0005] In both of the above processes, a resulting non-uniform microstructure can cause the metal matrix composite to have physical properties, e.g., thermal conductivity, that are compromised. Improvements in microstructure have been achieved by adding at least one sintering aid, such as cobalt, nickel or iron, to the powder mixture before sintering. The sintering aid wets the reinforcement material and facilitates improved transport of the metal matrix. However, a substantial disadvantage of sintering aids is that they tend to reduce the thermal conductivity of the metal matrix composite even if present in the small amounts required to achieve improved densification and microstructure.

SUMMARY

[0006] A metal matrix composite containing copper and reinforcement material has improved density and microstructure when the metal matrix composite also contains a small amount of silver or gold. In one embodiment, the metal matrix composite preferably contains from about 50% to about 95% reinforcement material, from about 5% to about 50% copper, and at least about 0.02% silver or gold, based on the weight of the metal matrix composite. The metal matrix composite can contain both silver and gold, so long as at least one of silver and gold is present. The reinforcement material can be any material that has a higher melting temperature than copper. If the metal matrix composite is used as a substrate for heat-generating components such as semiconductors, the reinforcement material is preferably one which has a coefficient of thermal expansion that is substantially lower than copper. The metal matrix composite of the present invention shows a more consistent microstructure throughout its cross-section and provides improved physical and thermal properties.

[0007] The process for making a metal matrix composite, according to one embodiment, includes compacting powder particles of reinforcement material to form a green compact, sintering the green compact to form a porous skeletal body, and infiltrating the porous skeletal body with an infiltrant containing copper and silver or and gold, to produce a near net shape metal matrix composite. The presence of silver or gold in the infiltrant facilitates improved capillary infiltration of the porous skeletal body.

[0008] In another embodiment, the process for making a metal matrix composite without infiltrating includes mixing powder particles of reinforcement material, copper, and silver or gold to form a powder mixture; compacting the powder mixture to form a green compact; and sintering the green compact to form a near net shape metal matrix composite. The silver in the infiltrant and powder mixture above may be present as elemental silver or as a copper-silver alloy. Likewise, the gold may be present as elemental gold or as a copper-gold alloy. The infiltrant and powder mixture can contain both silver and gold, so long as one is present.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention may be more readily understood by reference to the following drawings and photomicrographs wherein:

[0010]FIG. 1 is a schematic flow diagram of the production of metal matrix composites according to one embodiment of the present invention;

[0011]FIG. 2 is a schematic flow diagram of the production of metal matrix composites according to another embodiment of the present invention;

[0012]FIG. 3 is a scanning electronic microscope (SEM) microphotograph of a copper/tungsten composite made by the process of infiltration according to the prior art;

[0013]FIG. 4 is a scanning electronic microscope (SEM) microphotograph of a copper/tungsten composite comprising about 0.05% by weight silver, according to one embodiment of the invention;

[0014]FIG. 5 is a scanning electronic microscope (SEM) microphotograph of a copper/tungsten composite comprising about 0.1% by weight silver, according to another embodiment of the invention; and

[0015]FIG. 6 is a scanning electronic microscope (SEM) microphotograph of a copper/tungsten composite comprising about 0.3% by weight silver according to another embodiment of the invention.

DETAILED DESCRIPTION

[0016] The present invention applies to metal matrix composites in which the physically continuous metallic constituent, i.e. the metal matrix, contains copper. Metal matrix composites containing copper are widely used as substrates, for example, in applications requiring good electrical and thermal conductivity. Researchers have observed that when copper is alloyed with other elements, for example, cobalt, iron and nickel, the presence of these other elements, even in small amounts, can deleteriously affect the thermal properties and the overall performance of the metal matrix composite. The present invention is based on the discovery that the presence of silver or gold in a metal matrix composite results in improved microstructure and thermal conductivity properties. Metal matrix composites herein can be made using powder metallurgy dry press and sintering processes, with or without infiltration. These processes are further described herein below.

[0017] A flow scheme of the process for making metal matrix composite parts according to several embodiments of the invention is can be described with reference to FIG. 1. In one flow scheme, particles of reinforcement material (A) and, optionally, binder (B) are transported to admixing station 10 where they are intimately admixed together. From admixing station 10, the admixture is then transferred to compaction station 20 where they are charged into a suitable mold and compacted or pressed to form a green compact. The green compact is then transported to a sintering station 30, where the green compact is sintered to form a porous skeletal body having a density that preferably ranges from about 50% to about 90% of theoretical density. The porous skeletal body is then infiltrated at infiltration station 40 with an infiltrant (C) comprising copper and silver or gold, to produce a near net shape metal matrix composite. The near net shape metal matrix composite may optionally undergo surface finishing at finishing station 50 or plating at plating station 60, or both finishing and plating, to produce the metal matrix composite.

[0018] In the process described above, the presence of silver or gold in infiltrant (C) improves the wetting of the porous skeletal body, and therefore, the flow of the infiltrant through the smallest and largest capillary openings of the porous skeletal body during infiltration. The silver may be present in the infiltrant as elemental silver or as a copper-silver alloy composition. The gold may be present in the infiltrant as an elemental gold or as a copper-gold alloy composition. Improved microstructure is observed when the infiltrant preferably contains at least about 0.1%, more preferably, from about 0.3% to about 20%, even more preferably, from about 0.4% to about 4%, and yet even more preferably, from about 0.7% to about 2.5% of silver or gold, based on the weight of the infiltrant. The infiltrant can contain both silver and gold, so long as at least one is present. When a metal matrix composite having excellent thermal properties is desired, the remainder of the infiltrant is preferably copper to produce a matrix metal that is free or nearly free of impurities. As such, the infiltrant contains, preferably, at least about 99.9%, more preferably, from about 80% to about 99.7%, even more preferably, from about 96% to about 99.6%, and yet even more preferably, from about 97.5% to about 99.3% copper, based on the weight of the infiltrant.

[0019] When silver or gold in infiltrant (C) forms or is introduced as a copper-silver alloy or a copper-gold alloy, respectively, it is found that the relatively low melting temperatures of such alloys facilitate improved densification during infiltration. The presence of a copper-silver or a copper-gold alloy, causes a portion of the infiltrant to melt at a lower temperature during infiltration. Although not wishing to be bound by any theory, it is believed that as the temperature is increased during infiltration, the portion of the infiltrant that melts at a lower temperature liquefies and thereby wets the reinforcement material of the porous skeletal body to facilitate improved infiltration of the remaining portion of the infiltrant.

[0020] Table 1 below shows the melting temperatures of copper, silver, copper-silver alloy that is an eutectic composition and copper-gold alloy that is an eutectic composition. Eutectic alloy compositions contain alloy components in a ratio that exhibits the lowest melting temperature of all possible alloy compositions. This information, as well as the copper-silver and copper-gold phase diagrams can be found in Hansen, Max and Anderko, Kurt, Constitution of Binary Alloys, Genium Publishing Corp., Third Printing, 1991, pp. 18-19, 198-201. The copper-silver eutectic composition of about 28% copper and about 72% silver has a melting temperature that is more than 300° C. less than the melting temperature of copper. The copper-gold eutectic composition of about 20% copper and about 80% gold has a melting temperature that is almost 200° C. less than copper. Phase diagrams of copper-silver and copper-gold alloys show that copper-silver alloy compositions and copper-gold alloy compositions other than the eutectic compositions have melting temperatures that are lower than copper. That is, several copper-silver alloy compositions and copper-gold alloy compositions other than the eutectic composition are also characterized by melting temperatures lower than each of the copper and silver elements. TABLE 1 Cu/Ag Cu/Au Material Copper (Cu) Silver (Ag) Gold (Au) Eutectic Eutectic Melting 1083° C. 960.5° C. 1063° C. 779° C. 889° C. Temp.

[0021] In accordance with one embodiment, silver is present in the infiltrant (C) in the form of copper-silver alloy. The copper-silver alloy composition can range from about 10% to about 40% copper and from about 90% to about 60% silver, preferably, from about 20% to about 35% copper and from about 80% to about 65% silver, more preferably, from about 25% to about 30% copper and 75% to about 77% silver, and even more preferably, the eutectic composition of about 28% copper and about 72% silver. In another embodiment, gold is present in infiltrant (C) in the form of copper-gold alloy. The copper-gold alloy can range from about 10% to about 40% copper and from about 60% to about 90% gold, preferably, from about 15% to about 30% copper and from about 70% to about 85% silver, more preferably, from about 15% to about 25% copper and from about 75% to about 85% gold, and even more preferably, the eutectic composition of about 20% copper and about 80% gold. It is desirable that the copper-silver alloy and the copper-gold alloy be as close to a pure alloy as possible, however, one skilled in the art will understand that such an alloy can contain trace amounts of impurities.

[0022] The mixing, compacting, sintering and infiltrating steps referred to in FIG. 1 are described in more detail below.

[0023] The particles of reinforcement material (A) and, optionally, particles of binder (B), are blended at admixing station 10 to form a homogenous mass suitable for compaction. This can be accomplished in any conventional manner. For example, the particles can be mixed by means of mechanical mixers such as high shear mixers, blenders and the like. The particles can also be mixed in various types of mills, such as ball mills, rod mills and so forth, using appropriate grinding media so as to avoid unwanted contamination.

[0024] A reinforcement material can be a single element, an alloy, a compound or any material that has a melting temperature greater than copper (1083° C.), preferably at least about 100° C. greater than copper, and is insoluble in copper under sintering and infiltration conditions. In applications in which it is required that the metal matrix composite have excellent dimensional stability, the reinforcement material can have a melting temperature that is several hundred degrees higher than the melting temperature of copper and a coefficient of thermal expansion (CTE) that is lower than copper. In electrical applications, for example, the reinforcement material preferably has a CTE that is about 7.5 ppm/° C. or less. Examples of materials which can be used as a reinforcement material include, but are not limited to, tungsten, molybdenum, iron-nickel alloy, such as, for example, Invar®, iron-nickel-cobalt alloy, such as, for example, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafnium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride and silicon nitride. In another embodiment of the invention, particles of reinforcement material (A) can be a mixture of two or more reinforcement materials. In such case, one or more reinforcement material can be optionally mixed with binder (B), compacted to form a green body, and sintered to form a porous skeletal body. The porous skeletal body is then infiltrated with infiltrant (C) to produce a near net shape metal matrix composite.

[0025] Binder (B) adheres the dry particles to adhere to one another so that they form a shape during compacting. The amount of binder used preferably ranges from about 0.5% to about 2% by weight of the reinforcement material and binder mixture. Binders for powder metallurgy are well known by those skilled in the art, and a suitable binder, for example, is Acrawax C from Lonza Inc. of Fairlawn, N.J.

[0026] The particle size of the reinforcement material powders used as raw materials is not are critical; however, as will be appreciated by those skilled in the art of powder metallurgy, the particle size and the particle size distribution of powders does have a bearing on the microstructure of the ultimate products obtained. The average particle size of raw material, including reinforcement material, ranges from about 0.3 to about 30 microns, preferably from about 2 to about 20 microns, more preferably from about 3 to about 15 microns, and even more preferably, from about 4 to about 12 microns. In the coarser particle sizes of greater than about 5 microns, a dry mix of reinforcement material, and optionally binder, or agglomerates of reinforcement material, and optionally binder, can be successfully compacted to form a green compact without an additional spray-drying operation (described below). As a result, the overall process can be accomplished at a lower cost.

[0027] In another embodiment, reinforcement material having an average particle size that is at least about 5 microns about can be made into agglomerates before being compacted. With reference to FIG. 1, reinforcement material (A), and optionally, binder (B) is mixed with liquid (D), such as, for example, water, at admixing station 10. The admixture can be subjected to high sheer mixing until essentially all of the liquid evaporates, thereby forming agglomerates as the product. If high sheer mixers or blenders, for example, a Patterson-Kelly Blender or a V-blender are used, in which case the amount of liquid present should be relatively low, e.g., about 0.1% to about 10% by weight, preferably, about 1% to about 4% by weight. The agglomerates so formed can be screened to remove lumps and foreign matter therefrom, if necessary. By forming agglomerates of the reinforcement material, the flowability of the material to be compacted can be improved when the mean particle size of the reinforcement material is less than about 2 microns. This enables the reinforcement material to fill the compaction die much more easily than possible with unagglomerated reinforcement material having small average particle size. A suitable binder that is used for spray-drying is Rhoplex B-60A sold by Rohm & Haas of Philadelphia, Pa. The agglomerates are then transferred to compaction station 20 where they are charged into a suitable mold and compacted or pressed to form a green compact.

[0028] In another embodiment, the reinforcement material can optionally undergo a spray-drying operation before being compacted. Spray-drying to form agglomerates is preferred when the average particle size of reinforcement materials is less than about 5 microns, although larger particles can be spray-dried. FIG. 2 illustrates another flow scheme in which admixed particles of reinforcement material (A) and binder (B) can be transferred from admixing station 10 to spray-dryer 70 where they are mixed in the presence of a liquid (D), such as for example, water. The formation of free flowing agglomerates can be accomplished in a variety of different ways. It is most easily accomplished by spray drying a liquid mixture of the reinforcement material. This may be accomplished in various types of milling equipment, preferably provided with the appropriate liners and grinding media to avoid contamination, in which case the liquid content is usually considerably higher, for example, about 40% to about 90% by weight, preferably about 60% to about 70% by weight. The remaining flow scheme of FIG. 2 illustrating the various process steps at compaction station 20, sintering station 30, infiltration station 40, finishing station 50 and plating station 60 is the same as that illustrated in FIG. 1.

[0029] The amount of reinforcement material that is used to form a green compact of a predetermined size depends on the desired composition of the metal matrix composite to be produced. When the metal matrix composite is made for microelectronic and optoelectronic applications, for example, the near net shape metal matrix composite comprises from about 50% to about 95%, preferably from about 60% to about 90% by weight, and more preferably from about 70% to about 90%, reinforcement material. The amount of reinforcement material used for making a green compact is calculated so that the near net shape metal matrix composite is preferably at least about 50% by weight reinforcement material to avoid pooling of the infiltrant during infiltration.

[0030] Compaction to form a green compact of a predetermined size is accomplished at compaction station 20 in accordance with any conventional technique, for example, by a hydraulic or mechanical press. Several processing variables, such as, for example, the particle size of the powdered reinforcement material and the compaction pressure, are controlled to achieve a desired porosity, or density, of the green compact.

[0031] In dry pressing, the compaction pressure affects the green density which in turn determines the composition of the final infiltrated product. The reinforcement material, optionally mixed with binder, is compacted to produce a green compact having a density that ranges from about 50% to about 90% of theoretical density, to allow for successful sintering and infiltration. For dry pressing, the particle sizes range from about 2 to about 20 microns, and the compaction pressure preferably ranges from about 15,000 psi to about 80,000 psi. Alternative techniques for compaction, such as, for example, isopressing, tape casting, and sedimentation, can also be used to make a green compact suitable for sintering.

[0032] After the green compact is removed from the press, it is further densified when it undergoes sintering at sintering station 30 to form a porous skeletal body. Sintering is preferably accomplished using a furnace, for example, a batch furnace or a continuous pusher-type furnace. The sintering temperature can vary according to the reinforcement materials employed, and typically, the sintering temperature ranges from about 60% to about 80% of the melting temperature of the reinforcement material. Sintering conditions suitable for forming tungsten and molybdenum skeletal bodies, for example, can be carried out at temperatures that range from about 1400° C. to about 1500° C., preferably from about 1430° C. to about 1470° C., and more preferably from about 1440° C. to about 1460° C. for time periods ranging from about 0.5 to about 2 hours, preferably from about 0.5 to about 1 hour, and more preferably from about 0.7 to about 1 hour. The sintering atmosphere preferably comprises hydrogen, or a mixture of nitrogen and hydrogen, preferably containing at least about 90% hydrogen, and has a dew point of at least about 0° C., and preferably at least about 10° C.

[0033] The porous skeletal body is then infiltrated at infiltration station 40 with infiltrant (C) to produce a near net shape metal matrix composite. The porous skeletal body is infiltrated by placing the infiltrant in contact with the porous skeletal body and by applying heat to the infiltrant so that it melts to form a liquid. The infiltrant can be prepared for infiltration by first mixing particles of copper and particles of silver or gold, preferably in the form of a copper-silver alloy or copper-gold alloy, respectively, as described above, to form a mixture. The infiltrant can be placed into contact with the porous skeletal body, for example, by placing the infiltrant directly on the porous skeletal body. Alternatively, the infiltrant can be compacted to form an infiltrant-preform or pellet, which can be placed into contact with the sintered skeletal body. The copper powder particles preferably have a mean particle size of about 30 microns or less, preferably, about 10 microns or less, and more preferably, about 5 microns or less. The particles of silver, gold, copper-silver alloy and copper-gold alloy, are better dispersed throughout the copper particles if they are smaller than the copper particles. Particles of silver, gold, copper-silver alloy and copper-gold alloy have a mean particle size that is preferably less than the copper particles, and preferably, about 5 microns or less, and more preferably, about 1 micron or less.

[0034] Infiltration conditions suitable for infiltrating an infiltrant comprising copper and at least one of silver and gold, for example, can be carried out at temperatures that range from about 1100° C. to about 1500° C., preferably from about 1150° C. to about 1300° C., and more preferably from about 1200° C. to about 1250° C. for time periods ranging from about 0.5 to about 1 hour and preferably, from about 0.7 to about 1 hour. The infiltration atmosphere preferably contains hydrogen, or a mixture of nitrogen and hydrogen, preferably containing at least about 90% hydrogen. The dew point of the infiltration atmosphere is preferably about 10° C. or less, more preferably about 0° C. or less, and even more preferably, about −20° C. or less.

[0035] The amount of infiltrant needed to achieve full infiltration, that is, at least about 97% of theoretical density, is dependent upon the density of the porous skeletal body. For example, the volume of infiltrant to be infiltrated can be calculated by considering the overall volume and porosity of the porous skeletal body. The porosity, or the density, of the porous skeletal body can be determined by the Achimedes Principle which is well known by those skilled in the art. A slight excess of infiltrant can be used to ensure full densification, and the excess infiltrant that solidifies as a raised surface on the substrate, if any is present at all, can be removed by conventional machining. The infiltration step completes the production of a near net shape metal matrix composite.

[0036] In another embodiment according to the flow scheme shown in FIG. 1, particles of copper (E) are mixed with reinforcement material (A), and optionally, binder (B) at admixing station 10 to form an admixture, before being compacted at compaction station 20 to form a green body. The green compact is then transported to sintering station 30 where the green compact is sintered to form a porous skeletal body. The porous skeletal body is then infiltrated with infiltrant (C) containing copper and at least one of silver and gold at infiltration station 40 to form a metal matrix composite. The infiltrant (C) can contain silver present as elemental silver or as a copper-silver alloy composition, or gold present as an elemental gold or as a copper-gold alloy composition. As described with respect to the embodiments described above, the infiltrant can contain both silver and gold, so long as at least one is present. The presence of copper in the green body wets the surface of the reinforcement material during sintering, thereby improving the flow of infiltrant through the capillary or voids of the porous skeletal body during infiltration. The amount of copper in the admixture can range from about 1% to about 10% by weight with the remainder being substantially reinforcement material.

[0037] In another embodiment shown in accordance with the flow scheme of FIG. 2, the particles of reinforcement material (A), copper (E), and optionally binder (B), can be mixed with water (D) and undergo spray-drying as described above before being pressed to form a green compact at compaction station 20. The green compact is then transported into sintering station 30 where the green compact is sintered to form a porous skeletal body. The porous skeletal body is then infiltrated with infiltrant (C), containing copper and silver or gold, at infiltration station 40 to form a metal matrix composite. In yet another embodiment, the porous skeletal body produced according to the flow schemes of either FIG. 1 or FIG. 2 are infiltrated with infiltrant (C) as described in any of the several embodiments described above.

[0038] In another embodiment referring to FIG. 1, particles of silver (F) are mixed with particles of the reinforcement material (A), and optionally, binder (B) at admixing station 10 to form an admixture. The mixture is then compacted at compaction station 20 to form a green compact. The green compact is then sintered at sintering station 30 to form a porous skeletal body and infiltrated at infiltration station 40 with infiltrant (C) which contains copper to form a near net shape metal matrix composite. The admixture to be compacted contains, by weight, preferably at least about 0.02%, more preferably from about 0.03% to about 10%, even more preferably from about 0.05% to about 5%, and yet even more preferably from about 0.1% to about 1.5% silver. The admixture can contain silver present as elemental silver or as a copper-silver alloy composition. The admixture can contain both silver and gold, so long as at least one is present. In one embodiment, the silver is present in the admixture in the form a copper-silver alloy and the copper-silver alloy composition ranges from about 10% to about 40% copper and from about 90% to about 60% silver, preferably, from about 20% to about 35% copper and from about 80% to about 65% silver, more preferably, from about 25% to about 30% copper and 75% to about 70% silver, and even more preferably, the eutectic composition of about 28% copper and about 72% silver. It is understood that a copper-silver alloy contains a homogenous mixture of substantially copper and silver, however, one skilled in the art will appreciate that an alloy of the above compositions can contain trace amounts of impurities. The porous skeletal body produced upon sintering, which contains reinforcement material and silver, is then infiltrated with infiltrant (C) containing substantially copper. In another embodiment the infiltrant contains copper and silver or gold, as described above.

[0039] In another embodiment shown in accordance with the flow scheme of FIG. 2, the particles of reinforcement material (A), silver (F), and optionally binder (B), can be mixed with water (D) and undergo spray-drying as described above before being pressed to form a green compact at compaction station 20. The green compact is then transported into sintering station 30 where the green compact is sintered to form a porous skeletal body. The porous skeletal body is then infiltrated with infiltrant (C) containing substantially copper at infiltration station 40 to form a metal matrix composite. In yet another embodiment, the porous skeletal body produced according to the flow schemes of either FIG. 1 or FIG. 2 are infiltrated with infiltrant (C) containing at least about 0.1% by weight silver, preferably in the form of a copper-silver alloy. That is, silver is introduced in both the admixture to be compacted as well as in the infiltrant.

[0040] In another embodiment, particles of gold (G) are mixed with the reinforcement material (A) and optionally binder (B), at admixing station 10 to form an admixture. The admixture is then compacted at compaction station 20 to form a green compact. The green compact is then sintered at sintering station 30 to form a porous skeletal body and infiltrated at infiltration station 40 with infiltrant (C) which contains copper to form a near net shape metal matrix composite. The admixture includes, by weight, preferably at least about 0.02%, more preferably, from about 0.03% to about 10%, even more preferably from about 0.05% to about 5%, and yet even more preferably from about 0.1% to about 1.5% gold. The admixture can contain gold present as elemental gold or as a copper-gold alloy composition. The admixture can contain both silver and gold, so long as at least one is present. In one embodiment, the gold present in the admixture is in the form of a copper-gold alloy. The copper-gold alloy composition can range from about 10% to about 40% copper and from about 90% to about 60% gold, preferably, from about 20% to about 35% copper and from about 80% to about 65% gold, more preferably, from about 25% to about 30% copper and 75% to about 70% gold, and even more preferably, the eutectic composition of about 20% copper and about 80% gold. It is understood that a copper-gold alloy contains a homogenous mixture of substantially copper and gold, however, one skilled in the art will appreciate that an alloy of the compositions described above can contain trace amounts of impurities. The porous skeletal body produced upon sintering, containing reinforcement material and gold, is infiltrated with infiltrant (Cy containing substantially copper. In another embodiment the infiltrant contains copper and silver or gold, as described above.

[0041] In another embodiment shown in accordance with the flow scheme of FIG. 2, the particles of reinforcement material (A), gold (F) preferably in the form of a copper-gold alloy, and optionally binder (B), can be mixed with water (D) and undergo spray-drying as described above before being pressed to form a green compact at compaction station 20. The green compact is then transported into sintering station 30 where the green compact is sintered to form a porous skeletal body. The porous skeletal body is then infiltrated with infiltrant (C) containing substantially copper at infiltration station 40 to form a metal matrix composite. In yet another embodiment, the porous skeletal body produced according to the flow schemes of either FIG. 1 or FIG. 2 are infiltrated with infiltrant (C) containing at least about 0.1% by weight gold, preferably in the form of a copper-gold alloy. That is, gold is introduced in both the admixture to be compacted as well as in the infiltrant.

[0042] In another embodiment, metal matrix composites having an improved microstructure can be made using dry powder pressing without infiltration. In this embodiment, the sintering step produces a near net shape metal matrix composite rather than a porous skeletal body. Referring to FIG. 1, in one embodiment of the invention the particles of reinforcement material (A), optionally binder (B), particles of copper (E) and silver (F) or gold (G) are transported to admixing station 10 where they are intimately admixed together to form an admixture. The infiltrant can contain both silver and gold, so long as at least one is present. The silver may be present in the infiltrant as elemental silver or as a copper-silver alloy composition. The gold may be present in the infiltrant as an elemental gold or as a copper-gold alloy composition. From admixing station 10, the admixture is then transferred to compaction station 20 where it is charged into a suitable mold and compacted or pressed to form a green compact. The green compact is then transported into a sintering station 30, where the compact is sintered to produce a near net shape metal matrix composite. The near net shape metal matrix composite may optionally undergo surface finishing at finishing station 50 or plating at plating station 60, or both finishing and plating, to produce the metal matrix composite.

[0043] In one embodiment of the invention, the admixture to be compacted contains, by weight, preferably at least about 0.02%, more preferably from about 0.03% to about 10%, even more preferably from about 0.05% to about 5%, and yet even more preferably from about 0.1% to about 1.5% silver. In another embodiment of the invention, the admixture includes, by weight, preferably at least about 0.02%, more preferably, from about 0.03% to about 10%, even more preferably from about 0.05% to about 5%, and yet even more preferably from about 0.1% to about 1.5% gold.

[0044] In another embodiment of the invention, the silver is in the form of a copper-silver alloy composition that can range from about 10% to about 40% copper and from about 90% to about 60% silver, preferably, from about 20% to about 35% copper and from about 80% to about 65% silver, more preferably, from about 25% to about 30% copper and 75% to about 70% silver, and even more preferably, the eutectic composition of about 28% copper and about 72% silver. In another embodiment of the invention, the gold is in the form of a copper-gold alloy composition that can range from about 10% to about 40% copper and from about 90% to about 60% gold, preferably, from about 15% to about 30% copper and from about 85% to about 70% silver, more preferably, from about 15% to about 25% copper and from about 85% to about 75% gold, and even more preferably, the eutectic composition of about 20% copper and about 80% gold. It is desirable that the copper-silver alloy and the copper-gold alloy be as close to a pure alloy as possible, however, one skilled in the art will understand that an alloy can contain trace amounts of impurities.

[0045] In another embodiment, the reinforcement material, copper and silver or gold, having an average particle size that is at least about 5 microns, about can be made into agglomerates before being compacted. The admixture can be subjected to high sheer mixing in the presence of water until essentially all of the liquid evaporates, thereby forming agglomerates as the product. In another embodiment, in accordance with FIG. 2, particles of the raw materials can be transferred from admixing station 10 to undergo a spray-drying operation at spray-dryer 70 as described above. From spray-dryer 70, the spray-dried agglomerates are transferred to compacting station 20 and compacted to form a green compact. The green compact is then transported into a sintering station 30, where the compact is sintered to produce a near net shape metal matrix composite.

[0046] A metal matrix composite produced according to any of the many embodiments described above, comprises by weight from about 5% to about 50%, preferably from about 10% to about 50%, and more preferably from about 10% to about 30%, and even more preferably, about 10% to about 20% copper, from about 50% to about 95%, preferably from about 50% to about 90% by weight, and more preferably from about 70% to about 90%, and even more preferably, from about 80% to about 90% reinforcement material; and at least about 0.02%, preferably from about 0.03% to about 10%, more preferably from about 0.05% to about 5%, and even more preferably from about 0.1% to about 1.5% of at least one of silver and gold.

[0047] The amount of copper depends upon the desired properties of the metal matrix composite. For example, in applications where the metal matrix composite is to be used as a substrate to dissipate heat, the copper content preferably comprises from about 10% to about 50% by weight copper. As an example of thermal performance, a metal matrix composite that has from about 10% to about 50% by weight copper has a thermal conductivity that ranges from about 190 W/mK to about 250 W/mK. In applications where the metal matrix composite is required to have a coefficient of thermal expansion (CTE) of about 7.5 ppm/° C. or less, and a thermal conductivity of at least about 190 W/mK, the copper content is preferably from about 10% to about 15% by weight copper.

[0048] The final composition of a metal matrix composite made according to the present invention can be controlled by altering the processing variables such as powder particle size, compaction pressure, and the sintering and infiltration conditions (if made using infiltration), described above. As will be understood by those skilled in the art, no particular combination of processing conditions is an absolute requirement, rather these factors are guides to arrive at a metal matrix composite having a particular composition and improved microstructure.

[0049] In any of the processes described above, the near net shape metal matrix composite may undergo secondary operations. For example, a near net shape metal matrix composite that is made via infiltration, may undergo surface finishing at finishing station 50. The infiltrated near net shape metal matrix composites can be ground, tumbled and/or machined to the required dimensions and surface finish. They can be ground to clean off the excess copper on the surface and the finishing can smooth off sharp edges and eliminate burrs.

[0050] Depending upon the end use application, the near net shape metal matrix composite can undergo plating at plating station 60 to provide, on one or more surfaces, a secondary metallic coating. This can be done, for example, by plating with nickel using conventional plating processes such as electroless nickel plating, electroplating or the like. Electroless nickel is preferred because is produces a dense, uniform coating. Metal-coated metal matrix composites find wide applications in electronic packaging. If desired, such metal matrix composites can be further plated with other metals such as gold, copper or silver or combinations thereof.

[0051] In order to more fully and clearly describe the present invention so that those skilled in the art may better understand how to practice the present invention, the following examples are given. These examples are intended to illustrate the invention and should not be construed as limiting the invention disclosed and claimed herein in any manner.

WORKING EXAMPLES Examples 1-3

[0052] Metal matrix composites comprising copper and tungsten were made according to a prior art method in Examples 1 through 3. Tungsten particles from Teledyne Advanced Materials of Huntsville, Ala. were mixed with acrylic binder sold under the product name Rhoplex B-60A from Rohm & Haas of Philadelphia, Pa., and spray-dried to form agglomerates. The metal matrix composites in Examples 1 and 2 were made using two different lots of powder tungsten, and both lots had a mean particle size of 5 microns. The metal matrix composite in Example 3 was made using powder tungsten that had a mean particle size of 17 microns. In making each sample, approximately 5 grams of the spray-dried powder was placed into a mold and pressed at 80 ksi to obtain a green compact having a density that ranged from approximately 13-14 g/cc. The green compact was sintered in a hydrogen/nitrogen atmosphere at 1450° C. for one hour to produce a porous skeletal body. The porous skeletal body was then infiltrated with 0.88 grams of high conductivity copper (oxygen-free), available from Clark Company of Smithfield, R.I. and sold as Clark Mini Fastener C102 and pressed before infiltration.

[0053] The composition and physical properties of the metal matrix composites produced, i.e. the infiltrated density, the thermal conductivity, and visible microstructure are listed in Table 2 below. Density was measured per the Achimedes Principle. Thermal conductivity was determined by the Laser Flash Method ASTM E 1461-92. Microstructural evaluation was done by scanning electron microscopy (SEM) at 1,000 magnification to determine the grain size of the reinforcement material, and the presence or absence of voids, fractured surfaces and copper pools. The resulting metal matrix composites each had a relative density that ranged between 97% and 98% of the theoretical density, however, the microstructures of all three metal matrix composites were porous. The SEM photomicrograph of Example 1 is shown in FIG. 3. and reveals a non-uniform microstructure which is evidence of the poor wettability or the inability to achieve complete capillary infiltration.

Examples 4-8

[0054] In Examples 4-8 samples of copper/tungsten metal matrix composites were made in accordance with one embodiment of the present invention. Approximately 454 grams of tungsten powder particles having a mean particle size of 5 microns were mixed with 11.6 grams of acrylic binder and spray-dried to form agglomerates. In making each metal matrix composite, approximately 5 grams of spray-dried powder was placed into a 0.5 inch by 0.5 inch mold and was pressed at 80 ksi to obtain green compacts having a density ranging from 13-14 g/cc, which is approximately 67% to 73% of the theoretical density. The green compacts were sintered in a hydrogen/nitrogen atmosphere at 1450° C. for one hour to produce a porous skeletal body.

[0055] Infiltrants were prepared by mixing high-conductivity copper powder with particles of copper-silver eutectic alloy (minus 325 mesh) sold under the product name CuSil® from Willams Advanced Materials (WAM) of Buffalo, N.Y. The infiltrants in examples 4-8 contained 0.5%, 1%, 3%, 5% and 14% by weight copper-silver alloy of eutectic composition, respectively, which is approximately 0.36%, 0.72%, 2.2%, 3.6% and 10% by weight silver, respectively, based on the weight of the infiltrant. The infiltrant was pressed at approximately 5 ksi and placed on the porous skeletal body for infiltration in a BTU furnace with hydrogen atmosphere at 1250° C. for 45 minutes. The resulting metal matrix composites in examples 4, 5, 6, 7 and 8 contained 0.05%, 0.1%, 0.3%, 0.5%, and 1.4% by weight silver, respectively.

[0056] Properties of the metal matrix composites are shown in Table 2. Infiltrated density was measured per the Achimedes Principle, thermal conductivity was determined by the Laser Flash Method, and microstructural evaluation was done by scanning electron microscopy (SEM) at 1000 magnification. The photomicrographs of FIG. 4, FIG. 5, and FIG. 6, show microstructures of metal matrix composites that contain 0.05%, 0.1% and 0.3% by weight silver, respectively. Improved microstructure was achieved where the infiltrant contained 0.5%, 1%, and 3% by weight copper-silver alloy of the eutectic composition, (i.e. 0.36%, 0.72% and 2.2% by weight silver), respectively. The microstructure of the metal matrix composite that contains 0.05% by weight silver is visibly as good as the microstructure of the metal matrix composite that contains 0.3% by weight silver. The results show that higher silver concentrations can be used, but may unnecessarily add to the cost of the metal matrix composite. TABLE 2 Infiltrant Infiltrated Relative Exam- Cu Cu/Ag Density Density TC ple (wt. %) (wt. %) (g/cc) % w/mk Microstructure 1 15.2 — 16.03 98 206 unacceptable 2 15.3 — 16.15 98 207 unacceptable 3 14.8 — 16.08 97 196 unacceptable 4 15.58 0.5 16.18 99 208 good 5 16.43 1.0 16.1 99 211 good 6 14.9 3.0 16.11 99 203 good 7 14.7 5.0 16.21 99 203 good 8 13.8 14 16.23 99 212 good

Examples 9-24

[0057] Additional copper/tungsten metal matrix composite were made to determine the reproducibility in making metal matrix composites containing 0.5% copper-silver eutectic alloy, i.e., 0.36% by weight silver, based on the weight of the infiltrant. Tungsten spray-dried agglomerates and infiltrants, which contained silver in the form of copper-silver eutectic alloy, were made as described above with respect to Examples 4-8. Four composites were made on four different days, producing a total of 16 metal matrix composites. Table 3 lists the density of each porous skeletal body and the infiltrated density of each composite. Density was measured using Achimedes Principle. The thermal conductivity was determined by the Laser Flash Method. The results show that each metal matrix composite had a relative density that was at least 99% of the theoretical density, and microstructural evaluation of each composite at 1000 magnification confirmed that all of the metal matrix composites had good microstructure. The thermal conductivity of each composite was at least 192 W/mK. TABLE 3 Porous Infiltrated Copper Relative Thermal Skeletal Body Density Content Density Conductivity Example Density (g/cc) (g/cc) (%) (%) W/mK 9 14.20 16.59 14.33 100 200 10 14.19 16.52 14.08 99.5 198 11 14.17 16.51 13.86 99 204 12 14.15 16.50 13.85 99 199 13 13.88 16.46 14.37 99.5 203 14 13.94 16.51 13.88 99.2 203 15 13.82 16.38 14.28 99 198 16 13.90 16.39 14.34 99 196 17 13.87 16.47 14.26 99.4 206 18 13.94 16.48 14.35 99.5 205 19 13.92 16.46 13.76 99 192 20 13.99 16.48 13.92 99.1 200 21 13.95 16.45 14.02 99 204 22 13.87 16.41 14.42 99.2 206 23 13.86 16.46 14.47 99.5 194 24 13.94 16.44 14.38 99.3 202 25 15.6 17.2 10 99.4 191 26 12.3 15.6 20 99.5 219

Examples 25-26

[0058] A copper/tungsten metal matrix composite containing approximately 10% copper and a copper/tungsten metal matrix composite containing approximately 20% copper were produced using the same infiltration process described above in Examples 4-24. Approximately 454 grams of tungsten powder particles having a mean particle size of 5 microns were mixed with 11.6 grams of acrylic binder and spray-dried to form agglomerates. In Example 25, approximately 5 grams of spray-dried powder was placed into a 0.5 inch by 0.5 inch mold and was pressed at 118 ksi to obtain a green compact having an approximate density of 15.6 g/cc, or approximately 80% of theoretical density. In Example 26, approximately 5 grams of spray-dried powder was placed into a 0.5 inch by 0.5 inch mold and was pressed at 60 ksi to obtain a green compact having a density approximately 12.3 g/cc, which is about 64% of theoretical density. The green compacts were sintered in a hydrogen/nitrogen atmosphere at 1450° C. for one hour to produce a porous skeletal bodies which were then infiltrated with an infiltrant containing 0.5% by weight copper-silver alloy of the eutectic composition, (i.e. 0.36% silver) based on the weight of infiltrant, and infiltrated in a BTU furnace at 1250° C. for 45 minutes. The copper concentration, relative density, and thermal conductivity measured according to the methods described in examples 4-24 above, are listed in Table 3. Microstructural evaluation of each composite at 1000 magnification confirmed that the metal matrix composites of examples 25 and 26 had good microstructure.

[0059] All such modifications and variations of the present invention are possible in light of the above teachings. For example, additives and other modifying agents may be added to the metal matrix composite of the present invention. It is understood, however, that changes may be made in the particular embodiments described above which are within the full intended scope of the invention as defined in the appended claims. 

We claim:
 1. A process for making a metal matrix composites comprising: compacting powder particles of reinforcement material to obtain a green compact; sintering the green compact to obtain a porous skeletal body; and infiltrating the porous skeletal body with an infiltrant comprising copper and at least about 0.1% of silver or gold, based on the weight of the infiltrant, to obtain a near net shape matrix metal composite.
 2. The process of claim 1 wherein the infiltrant comprises at least about 0.1% of silver or gold based on the weight of the infiltrant.
 3. The process of claim 1 wherein the infiltrant comprises silver and the silver is present in the form of a copper-silver alloy.
 4. The process of claim 3 wherein the copper-silver alloy comprises from about 10% to about 40% by weight copper with the remainder being substantially silver.
 5. The process of claim 3 wherein the copper-silver alloy is a copper-silver eutectic composition.
 6. The process of claim 1 wherein: the infiltrant comprises gold and the gold is present in the form of a copper-gold alloy.
 7. The process of claim 6 wherein the copper-gold alloy comprises from about 10% to about 40% by weight copper with the remainder being substantially gold.
 8. The process of claim 6 wherein the copper-gold alloy is a copper-gold eutectic composition.
 9. The process of claim 1 wherein: the infiltrant comprises at least about 0.3% silver present in the form of copper-silver alloy, based on the weight of infiltrant, with the remainder of the infiltrant being substantially copper.
 10. The process of claim 1 wherein the reinforcement material has a melting temperature of at least about 1180° C.
 11. The process of claim 1 wherein the reinforcement material is selected from the group consisting of tungsten, molybdenum, iron-nickel alloy, Invar®, iron-nickel-cobalt alloy, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafnium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride, silicon nitride and mixtures thereof.
 12. The process of claim 1 wherein the reinforcement material is at least one of tungsten and molybdenum.
 13. The process of claim 1 wherein the particles of reinforcement material have a mean particle size ranging from about 2 microns to about 20 microns.
 14. The process of claim 1 wherein the particles of reinforcement material are compacted to form green compact having a density that ranges from about 50% to about 95% of theoretical density.
 15. The process of claim 1 wherein the green compact is sintered in a gas atmosphere comprising hydrogen and having a dew point of at least about 0° C.
 16. The process of claim 15 wherein the green compact is sintered at a temperature ranging from about 1400° C. to about 1500° C. for about 0.5 hours to about 2 hours.
 17. The process of claim 1 wherein the porous skeletal body produced upon sintering has a density ranging from about 50% to about 95% of theoretical density.
 18. The process of claim 1 wherein the porous skeletal body is infiltrated in a gas atmosphere having a dew point of about 10° C. or less.
 19. The process of claim 1 wherein the porous skeletal body is infiltrated at a temperature ranging from about 1100° C. to about 1500° C.
 20. The process of claim 1 wherein the near net shape metal matrix composite comprises from about 5% to about 50% copper, from about 50% to about 95% reinforcement material, and at least about 0.05% by weight of silver.
 21. The process of claim 1 wherein the powder particles of the reinforcement material are mixed with powder particles of copper before being compacted to form a green compact.
 22. The process of claim 1 wherein the powder particles of the reinforcement material are mixed with powder particles of silver before being compacted to form a green compact prior to sintering the green compact.
 23. A process for making a matrix metal composites comprising: compressing powder particles of a reinforcement material to obtain a green compact; sintering the green compact to obtain a porous skeletal body; and infiltrating the porous skeletal body with an infiltrant comprising copper and at least about 0.1% of silver or gold, based on the weight of the infiltrant, in the form of a copper-silver alloy, the silver being in form of a copper-silver alloy and the gold being in the form of a copper-gold alloy, to obtain a near net shape matrix metal composite.
 24. The process of claim 23 wherein: the reinforcement material is selected from the group consisting of tungsten, molybdenum, iron-nickel alloy, Invar®, iron-nickel-cobalt alloy, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafnium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride and silicon nitride and mixtures thereof; the infiltrant comprises at least about 0.1% copper-silver alloy, based on the weight of infiltrant, with the remainder of the infiltrant being copper; and the copper-silver alloy comprises, by weight, from about 60% to about 90% silver with the remainder being substantially copper.
 25. The process of claim 24 wherein the copper-silver alloy is the eutectic composition.
 26. The process of claim 25 wherein the reinforcement material is tungsten or molybdenum.
 27. The process of claim 23 wherein: the reinforcement material is selected from the group consisting of tungsten, molybdenum, iron-nickel alloy, Invar®, iron-nickel-cobalt alloy, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafnium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride, silicon nitride and mixtures thereof; and the infiltrant comprises at least about 0.1% copper-gold alloy, based on the weight of infiltrant, with the remainder being copper; and the copper-gold alloy comprises, by weight, from about 60% to about 90% gold with the remainder being substantially copper.
 28. The process of claim 26 wherein the copper-gold alloy is the eutectic composition.
 29. The process of claim 28 wherein the reinforcement material is tungsten or molybdenum.
 30. The process of claim 23 wherein: the green compact is sintered in a gas atmosphere comprising hydrogen and having a dew point of at least about 0° C. to produce a porous skeletal body having a density ranging from about 50% to about 95% of theoretical density.
 31. The process of claim 23 wherein: the porous skeletal body is infiltrated in a gas atmosphere having a dew point of about 10° C. or less.
 32. The process of claim 31 wherein the porous skeletal body is infiltrated at a temperature ranging from about 1100° C. to about 1500° C.
 33. The process of claim 31 wherein: the infiltrant comprises at least about 0.1% silver in the form of copper-silver alloy, based on the weight of infiltrant, the copper-silver alloy comprising, by weight, from about 60% to about 90% silver and the remainder being substantially copper; and the near net shape metal matrix composite comprises, by weight, from about 5% to about 50% copper, from about 50% to about 95% reinforcement material, and at least about 0.02% silver.
 34. The process of claim 31 wherein: the infiltrant comprises from about 0.3% to about 20% silver in the form of copper-silver alloy, based on the weight of infiltrant, the copper-silver alloy comprising, by weight, from about 60% to about 90% silver and the remainder being substantially copper; and the near net shape metal matrix composite comprises, by weight, from about 10% to about 20% copper, from about 70% to about 90% reinforcement material, and from about 0.03% to about 10% silver; and the reinforcement material is tungsten or molybdenum.
 35. A process for making a metal matrix composites comprising: forming a mixture comprising, by weight, from about 5% to about 50% copper, from about 50% to about 95% reinforcement material, and at least about 0.02% of at least one of silver in the form of a copper-silver alloy or gold in the form of a copper-gold alloy, to form a mixture; compressing the mixture to obtain a green compact; and sintering the green compact to obtain a near net shape matrix metal composite.
 36. The process of claim 35 wherein: the mixture comprises from about 0.03% to about 10% by weight silver in the form of a copper-silver alloy composition, wherein copper-silver alloy composition comprises from about 10% to about 40% copper and from about 60% to about 90% silver; the reinforcement material is selected from the group consisting of tungsten, molybdenum, iron-nickel alloy, Invar®, iron-nickel-cobalt alloy, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafiium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride, silicon nitride and mixtures thereof; and the green compact is sintered in a gas atmosphere comprising hydrogen and having a dew point of at least about 0° C.
 37. The process of claim 35 wherein: the mixture comprises from about 0.03% to about 10% by weight gold in the form of a copper-gold alloy composition, wherein the copper-gold alloy composition comprises from about 10% to about 40% copper and from about 90% to about 60% gold; the reinforcement material is selected from the group consisting of tungsten, molybdenum, iron-nickel alloy, Invar®, iron-nickel-cobalt alloy, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafnium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride, silicon nitride and mixtures thereof; and the green compact is sintered in a gas atmosphere comprising hydrogen and having a dew point of at least about 0° C.
 38. A metal matrix composite comprising, by weight: at least about 0.03% silver or gold; from about 5% to about 50% copper; and from about 50% to about 95% reinforcement material.
 39. The metal matrix composite of claim 38 wherein the metal matrix composite comprises from about 0.03% to about 10% by weight silver.
 40. The metal matrix composite of claim 38 wherein the metal matrix composite comprises from about 0.03% to about 10% by weight gold.
 41. The metal matrix composite of claim 38 wherein the reinforcement material has a melting point of at least about 100° C. greater than the melting point of copper.
 42. The metal matrix composite of claim 38 wherein the reinforcement material is selected from the group consisting of: tungsten, molybdenum, iron-nickel alloy, Invar®, iron-nickel-cobalt alloy, Kovar®, tantalum, chromium, osmium, ruthenium, rhenium, rhodium, hafnium, zirconium, nickel, iron, cobalt, titanium, titanium carbide, tungsten carbide, tantalum carbide, chromium carbide, silicon carbide, beryllium oxide, aluminum oxide, boron nitride, aluminum nitride, silicon nitride and mixtures thereof.
 43. The metal matrix composite of claim 38 wherein the metal matrix composite comprises from about 0.05% to about 5% by weight silver.
 44. The metal matrix composite of claim 38 wherein the metal matrix composite comprises from about 0.05% to about 5% by weight gold.
 45. The metal matrix composite of claim 38 wherein the metal matrix composite comprises by weight: from about 0.05% to about 5% silver; from about 10% to about 30% copper; and from about 70% to about 90% reinforcement material.
 46. The metal matrix composite of claim 38 wherein the metal matrix composite comprises by weight: from about 0.05% to about 5% gold; from about 10% to about 30% copper; and from about 70% to about 90% reinforcement material.
 47. The metal matrix composite of claim 45 wherein the reinforcement material comprises at least one of tungsten and molybdenum.
 48. The metal matrix composite of claim 46 wherein the reinf orcement material comprises at least one of tungsten and molybdenum.
 49. The metal matrix composite of claim 45 further comprising gold.
 50. The metal matrix composite of claim 46 further comprising silver. 